Taking over for Jimmy Kimmel last night, Kristen Bell announced, "I'm in charge tonight, so I can do whatever I want." That ended up being a "promposal," a la her hit movie "Frozen."

First, Bell picked two audience members "at random" to play a game called "The Fast and the Frozenest." Zoe was dressed as Elsa and Sarah as Anna -- "the better one," Bell joked. She then sent them out to Hollywood Blvd. to find a guy dressed like Kristoff.

As soon as the women dashed out the front door, Bell leveled with the audience that it was all a set-up: "Sarah's high school prom is coming up in a few weeks and one of her classmates, a guy named Michael, asked her to help 'prompose' to her." The other girl, an actress, "is probably halfway home by now," Bell said as we watched Sarah find a guy wearing a giant Kristoff head.

Bell admitted that the two had never even gone on a date before and that, "This could all go terribly wrong. But you gotta admit that it would be great TV, right?"

When Sarah returned with Kristoff, Michael took off the character's head and got Bell to sing, "Do You Want To Build a Snowman" but with new lyrics: "Do you want to be his prom date.... if you say no, he'll have to take his Mom!" Fortunately, Sarah said yes.

Watch the whole promposal here:

Jimmy Kimmel is taking the rest of the week off after announcing Monday night that his newborn son is recovering from heart surgery.